The present invention relates generally to communication networks and more specifically to the use of multiple access protocols in asynchronous communication networks.
In communication networks, asynchronous transmission of information is often the technique used when communicating information between one or more communication devices within a communication network. Asynchronous transmission is often used when low power devices make up the network. These low power devices can use a low communication duty cycle frame structure in order to minimize the amount of power used while not actively communicating with other network devices, but the use of a low communication duty cycle frame structure often implies that device availability is reduced. In wireless communication networks, a fundamental challenge is maintaining high availability communications while using low power wireless communication devices.
The neuRFon(trademark) device by Motorola is an example of a low power, low cost, small size, and simple wireless device. The neuRFon(trademark) device network is a zero-configuring, self-organizing, asynchronous network containing multiple neuRFon(trademark) devices. For this network, power consumption and cost are two major concerns.
To lower the power consumption, the average communication duty cycle of all the devices in the described network has to be decreased to a minimum. The average communication duty cycle refers to the fraction of time that the wireless device is able to send and receive messages. For the described asynchronous network, the average communication duty cycle may be set so low that the infrequent communications between a transmitter and a destined receiver become a problem. For example, device A may attempt to contact device B, but device B may be not be able to receive messages due to its low average communication duty cycle. This will prevent device A from establishing contact.
A representative low average communication duty cycle frame structure, in which the above problem is illustrated, is shown in FIG. 1. Using this frame structure, a low average communication duty cycle device uses 1 ms to warm up, 1 ms to transmit and receive messages from other devices in its group, and is asleep for the remaining 998 ms of the 1 second cycle. This gives a communication duty cycle of about 0.1%, which is very power efficient.
The problem with this approach is illustrated in FIG. 2. FIG. 2 shows a small network comprising several low power, low average communication duty cycle devices, where each device is represented as a small dot. Referring again to FIG. 2, device A tries to talk to device B. If the low average communication duty cycle frame structure in FIG. 1 is assumed, both A and B devices are able to communicate only 0.1% of the time. If we further make the reasonable assumption that device A and device B don""t each have access to the other""s time schedule, the probability of device A to establish communication with device B is approximately 0.1%, which is too small for most applications.
To reduce the cost, low cost crystal and/or Micro Electro-Mechanical Systems (MEMS) may be utilized as the frequency generator for the wireless device. The issue with these technologies is their inherent poor frequency stability that further makes synchronization difficult. Removing the requirement for highly accurate frequency synchronization would allow the cost of such devices to be kept low. Such a protocol would enable asynchronous networks employing these low power, low communication duty cycle devices to be built at low cost and low power consumption.
An invention that introduces a new protocol enabling low power, low communication duty cycle devices to communicate with each other, while at the same time not requiring highly accurate frequency synchronization would be able to address the issues described above.